New 40Ar/39Ar Ages, Biostratigraphic and Geochemical Data from the Sabzevar Ophiolite, North Central Iran: Implications for Tectonic of Iranian Plate

The igneous rocks of the Sabzevar ophiolite in north central Iran composed of peridotites, serpentinite, minor pyroxenite, gabbros, and a volcanic sequence that exhibits a wide range of composition from basalts to basaltic andesites to rhyodacite-dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Lower Cretaceous deep- and shallow-marine rocks. These include pelagic fossiliferous carbonates, which are mixed with the pillow basalts and the basaltic andesite as interlayers or exotic blocks, ranging in size from 10 to 100 meters. Also present are extensive units of radiolarian chert which are interbedded within the basalts and basaltic andesites. A combination of petrographic observations and analyses of incompatible trace elements and rare earth elements indicates the presence of at least four different types of extrusive rocks in the Sabzevar ophiolite. The geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts; i) the group-1 basaltic rocks, which formed from an initial melt with N-MORB-like (LREE depleted) chemical signatures and are petrogenetically related to the gabbros, ii) group-2 basaltic rocks which have E-MORB chemical signatures, and iii) group-3 basaltic rocks with LREE-enriched signature and incompatible trace element patterns that suggest an island arc affinity. In addition, the Sabzevar ophiolite contains other volcanic rocks that have non-MORB like geochemical signatures (e.g., within-plate, island arc). The differences between these geochemical signatures could be a result of a differing composition of the upper mantle, or a different degree of partial melting of the same upper mantle. The presence of sizable Nb anomalies in the extended REE patterns for andesites and the group-2 basalts is characteristic of volcanic arc magmas, or those were the composition of the ascending magma was modified by crustal contamination. The presences of the basanites and the silica under saturated rocks could be attributed to post ophiolite emplacement extrusive activities. The most significant of the taxa recovered include Clobotruncanita calcarata (Zone 2B; uppermost Campanian)and Clobotruncanita contusa (Zone 1B, Sub-zone 1B1 to 1A: upper Maasrrichtian). The two most recent Ar/Ar ages from a muscovite and hornblende from two samples of schists give consistent plateau ages of 52.8 ñ 0.8 Ma and 52.97ñ 0.44 Ma, respectively. They have been interpreted as the ages of emplcement of this ophiolite and the age of the closure of the Tethys in this section of the present day Middle East. Plate reconstructions suggest that the rocks of Sabzevar ophiolite are part of the Tethyan oceanic crust that was formed during rifting of a narrow but deep seaway that separated the Central Iranian Microcontinent from the Eurasian plate and it was emplaced during northeast dipping subduction (i.e., closure) of the this segment of Tethyan seaway (Sabzevar Ocean).